Conventionally, semiconductor devices having what is called a MOS (Metal-Oxide-Semiconductor) structure, such as IGBT (Insulated Gate Bipolar Transistor) and MOSFET (MOS Field Effect Transistor), have been known. Such MOS-type semiconductor devices use overvoltage protection diodes made up of series-connected Zener diodes as overvoltage protection measures. More specifically, in the overvoltage protection diodes, n-type semiconductor layers and p-type semiconductor layers are alternately arranged adjacent to each other (e.g., see Patent Literature 1). In the case of IGBT, overvoltage protection diodes are provided between a collector terminal and a gate terminal and a gate terminal and an emitter terminal.
As shown in FIG. 8, a p-type semiconductor layer 50b (and n-type semiconductor layer) of an overvoltage protection diode are arranged on an insulating film 140 formed on a semiconductor substrate 120, and is covered with an insulating film 150.
Typically, in an overvoltage protection diode, the p-type impurity concentration in a p-type semiconductor layer is lower than the n-type impurity concentration in an n-type semiconductor layer. Consequently, the breakdown voltage (Zener voltage) of an overvoltage protection diode is determined according to the position of a high concentration region (concentration peak) of p-type impurity concentration. In a conventional overvoltage protection diode, as shown in FIG. 8, the p-type impurity concentration is the maximum in a boundary region F10 between the p-type semiconductor layer 50b and the insulating film 150. That is, the p-type impurity concentration in the boundary region F10 is higher than the p-type impurity concentration in an inner region G10. Consequently, the overvoltage protection diode causes Zener breakdown in the boundary region F10.